I got this error:
CD ROM's initialization.
Please install MSCDEX v 2.1 or above.

umm... So... Windows vista don't have that?

Nope, and neither does XP or Windows2000 :)

You have to launch the game through the shortcut to run it through DOSBOX, an emulator that does (among other things) contain MSCDEX.EXE :)

Thank you, but the problem persists, as I launched the game via DOSbox 0.74, it still gave me the same message.

That's... strange: What does it say on the first screen when you launch the game?

It shout say:

"Welcome to DOSBox
... The DOSBox Team http:...
===================
MSCDEX installed
Drive C is mounted as...
Drive D is mounted as...
"

What do you see?

Exactly as this:
"CD ROM's initialization.
Please install MSCDEX v 2.1 or above. "

with the ubisoft logo on the very top in a blue marquee bar...

But I did tried manually starting MSCDEX by inputing mount d C:\ -t cdrom at the end. Worked... but... at the expense of the lush music... Damn less nostalgic value now :(

Apparently you're not running the GOG version, or not through the preconfigured DOXBox (you should say that):

imgmount d "..\music\game.inst" -t iso -fs iso
MOUNT C "."
C:
RAYMAN.EXE

Oh ok i see, Its works when I open up the DOSbox GOG distributed, then type in the following... And boom MUSIC!!! Yes! Thanks now i can enjoy the game to the fullest! Thanks again!

I am having the exact same problem but I am completely new to DosBox and have no idea what I'm doing. I tried to implement what was recommended above to solve the problem but couldn't make heads or tales of how to actually do any of it. Could you explain the fix in the simplest manner possible?

Easiest is to either launch the game using the "Rayman Forever" shortcut which comes with the game or use the Rayman Control Panel to automatically configure these things. If you prefer to manually type in the DOSBox commands each time I can tell them, just let me know.